Modern society relies heavily upon electricity. With deregulation and privatisation, electricity has become a commodity as well as a means for competition. Power quality, as a consequence, is coming into focus to an extent hitherto unseen. Disturbances emanating from any particular load will travel far, and, unless properly remedied, spread over the grid to neighbouring facilities. A traditional way to deal with the problem of poor or insufficient quality of power distribution is to reinforce the grid by building new lines, installing new and bigger transformers, or moving the point of common coupling to a higher voltage level.
Such measures, however, are expensive and time-consuming, if they are at all feasible. A simple, straightforward and cost-effective way of power quality improvement in such cases is to install equipment especially developed of the purpose in the immediate vicinity of the source(s) of disturbance. As an additional, very useful benefit, improved process economy will often be attained enabling a profitable return on said investment.
Within flexible alternating current transmission systems (FACTS) a plurality of control apparatus are known. One such FACTS apparatus is the static compensator (STATCOM). A STATCOM comprises a voltage source converter (VSC) having an AC side connected to the AC network (transmission line) via an inductor in each phase. The DC side is connected to a temporary electric power storage means such as capacitors. In a STATCOM the voltage magnitude output on the AC side is controlled thus resulting in the compensator supplying reactive power or absorbing reactive power from the transmission line. With zero active power transfer, the voltage over the DC capacitors is constant when assuming that the converter losses are negligible. The VSC comprises at least six self-commutated semiconductor switches, each of which is shunted by a reverse or anti-parallel connected diode. A STATCOM apparatus with no active power source can only compensate for reactive power, balancing load currents and remove current harmonics in point of common connection by injecting current harmonics with opposite phase.
By bringing together STATCOM and IGBT (Insulated Gate Bipolar Transistor) technologies, a compact STATCOM with reactive power compensation is obtained which offer possibilities for power quality improvement in industry and power distribution. This performance can be dedicated to active harmonic filtering and voltage flicker mitigation, but it also allows for the compact STATCOM to be comparatively downsized, its footprint can be extremely small. The grid voltage profile may be controlled according to a given optimal characteristic, and the result is an enhanced grid capacity with a more stable, strengthened and predictable behavior. One example where the compact STATCOM, has proven to be very useful is in the steel making industry. An electric arc furnace (EAF) is a piece of equipment needed to make steel products. For the grid owner and for the supplier of electricity, the EAF user is a subscriber to power, i.e. a customer, but in the worst case also a polluter of the grid. Out of the EAF may well come an abundance of distortion such as voltage fluctuations, harmonics and phase asymmetry. Also, the grid may be subject to carrying large amounts of reactive power, which is unintended and gives rise to transmission and distribution losses as well as impedes the flow of useful, active power in the grid.
An electric arc furnace is a heavy consumer not only of active power, but also of reactive power. Also, the physical process inside the furnace (electric melting) is erratic in its nature, with one or several electrodes striking electric arcs between furnace and scrap. As a consequence, the consumption especially of reactive power becomes strongly fluctuating in a stochastic manner. The voltage drop caused by reactive power flowing through circuit reactances in the electrodes, electrode arms and furnace transformer therefore becomes fluctuating in an erratic way, as well. This is called voltage flicker and is visualized most clearly in the flickering light of incandescent lamps fed from the polluted grid.
The problem with voltage flicker is attacked by making the erratic flow of reactive power through the supply grid down into the furnaces decrease. This is done by measuring the reactive power consumption and generating corresponding amounts in the compact STATCOM and injecting it into the system, thereby decreasing the net reactive power flow to an absolute minimum. As an immediate consequence, voltage flicker is decreased to a minimum, as well.
Important added benefits are a high and constant power factor, regardless of load fluctuations over furnace cycles, as well as a high and stable bus RMS voltage. These benefits can be capitalized as improved furnace productivity as well as decreased operational costs of the process in terms of lower specific electrode and energy consumption and reduced wear on the furnace refractory.
To parry the rapidly fluctuating consumption of reactive power of the furnaces, an equally rapid compensating device is required. This is brought about with the state of the art power electronics based on IGBT technology. With the advent of such continuously controllable semiconductor devices capable of high power handling, VSCs with highly dynamic properties have become feasible far into the 100 MVA range.
The function of the VSC in this context is a fully controllable voltage source matching the bus voltage in phase and frequency, and with an amplitude which can be continuously and rapidly controlled, so as to be used as the tool for reactive power control.
The input of the VSC is connected to a capacitor, which is acting as a DC voltage source. At the outputs, the converter is creating a variable AC voltage. This is done by connecting the voltages of the capacitor or capacitors directly to any of the converter outputs using the valves in the VSC. In converters that utilise Pulse Width Modulation (PWM), the input DC voltage is normally kept constant when creating output voltages that in average are sinusoidal. The amplitude, the frequency and the phase of the AC voltage can be controlled by changing the switching pattern.
In the compact STATCOM, the VSC uses a switching frequency greater than 1 kHz. The AC voltage across the reactor at full reactive power is only a small fraction of the AC voltage, typically 15%. This makes the compact STATCOM close to an ideal tool for fast reactive power compensation.
For the compact STATCOM, the IGBT has been chosen as the most appropriate power device. IGBT allows connecting in series, thanks to low delay times for turn-on and turn-off. It has low switching losses and can thus be used at high switching frequencies. Nowadays, devices are available with both high power handling capability and high reliability, making them suitable for high power converters. Instead of the IGBTs another possibility is to use Gate Turn-Off thyristors (GTO), Integrated Gate Commutated Thyristors (IGCT), MOSFET or any self commutated device.
As only a very small power is needed to control the IGBT, the power needed for gate control can be taken from the main circuit. This is highly advantageous in high voltage converters, where series connecting of many devices is used. At series connection of IGBTs, a proper voltage division is important. Simultaneous turn-on and turn-off of the series connected devices are essential.
The converter topology for a compact STATCOM may be a two level configuration. In a two-level converter the output of each phase can be connected to either the positive pole or the negative pole of the capacitor. The DC side of the converter is floating, or in other words, insulated relative to ground. The two-level topology makes two numbers of output voltage combinations possible for each phase on the AC-side. One such converter topology is shown in FIG. 1.
An alternative to series connection of valve positions to achieve the necessary voltage rating is to connect converter cells in series. In this way smoother AC current and AC voltage waveforms are possible to obtain with lower switching frequency and minimal filtering. One such arrangement is series connection of single phase full-bridge converters, which sometimes are referred to as chain-link cells.
A chain-link based converter comprises a number of series-connected cell modules, each cell comprising a capacitor, besides the valves. The DC-capacitor of each such cell module is rather big compared to the above described two-level static compensator, when seen in relation to the total effect of the system.
A chain-link cell module may consists of four IGBT positions and a DC link Capacitor bank as shown schematically in FIG. 2. Each of the three VSC phases consists of a number of chain-link cells, here shown in series in the general diagram of FIG. 3 for a delta connected arrangement. The phases can also be connected in an Y-arrangement.
The number of cells in series in each phase is proportional to the AC voltage rating of the system and can, for high AC voltage systems, consequently include a large number of cells.
It is necessary with such high power systems with many cell modules in series to continue operation of the system with failed cell modules in circuit in order to achieve a reasonably high MTTR (Mean Time To Repair). To allow for this failure mode of operation the inventors have identified a number of requirements:                A number of redundant cell modules are needed to achieve the required MTTR figures.        The failed cell module must be bypassed in a safe way while the system is in operation.        The system is kept operational for the duration of the service interval despite failed cell modules.        The failed cell modules are then replaced during scheduled maintenance.        
To be able to bypass a faulty cell module, it is necessary to provide zero voltage across the AC terminals of the cell. This can be achieved by using a very fast mechanical switch or a solid-state relay (bidirectional thyristor) or a combination of the two above solutions to allow for low power losses as shown in FIG. 4 where these known solutions are illustrated.
One example of a device for protecting converter modules is disclosed in WO-2008/125494 where each submodule of the device is associated with a short circuit device, e.g. a vacuum switching tube, for short circuiting the submodule. The short circuit device enables safe bridging of a defective submodule.
The common features of these methods are that they require additional and controllable components to be introduced which inter alia adds on costs and complexity to the system.
Thus, the object of the present invention is to remove the above drawbacks.